I've whipped up a small plugin that looks for "modelines" in individual files onLoad and sets options locally to that file. Is there a built-in feature like this in SublimeText or is a plugin the right way to go?

Jon, I don't think there's a way of detecting whether a given name corresponds to a valid (built-in) option? I mean, as far as I understand, there are predefined application options which control certain built-in features and user-defined options with arbitrary names?